The general evolution of gas chromatographs has led to the use of very small glass capillary gas transfer lines and columns. A typical glass capillary tube has an inner diameter of 0.013 inch and 0.018 inch outer diameter. Vaporized chemical samples are carried, usually in a carrier gas, among the various devices in the gas chromatograph and between these devices and other devices located in adjacent detectors or other instruments external to the gas chromatograph through heated transfer lines.
The function of the transfer line heater is to maintain the vaporized chemical sample within defined temperature bounds as it is carried amoug the various components of the analytical instruments. A particular problem arises when the vaporized chemical samples are allowed to cool in the transfer lines, as this cooling can lead to precipitation of the sample out of the gas and onto the glass tube walls. Once this occurs, the chemical analysis is vitiated and there is the possibility that the glass tube can become plugged or that future chemical analysis can be contaminated. If heated above the temperature bounds, there is the possibility that the chemical sample will chemically react or decompose also vitiating the analysis. Therefore, it is important in gas chromatography that the glass capillary transfer lines be controllably heated.
Transfer lines are also used as gas conduits between analytical instruments, for example, between a gas chromatograph and a peripheral detector such as a spectrophotometer. For this use, the heated transfer line should be temperature controlled and powered in one instrument and protrude into the second. Since the small glass capillary tubes are difficult to clean in an instrument and are subject to breakage, the apparatus which heats the capillary tube must also permit its replacement.
In general, heated transfer lines are designed to operate in a temperature range of 150-350 degrees C., concurrent with that of present gas chromatographs. Since many chemical compounds have chemical reactivity or decomposition temperatures near their boiling points, it is highly desirable that the temperature range, or tolerance of the transfer lines be small, e.g. with 10 degrees C.
Several instrument oriented constraints on transfer line design have, heretofore, limited the temperature profile integrity of transfer lines. As a practical matter, it is desirable that a transfer line bridging two separate instruments be securely mounted, controlled and powered from one instrument while simply extending in to the second. The degree of mounting in the second instrument, if at all, may be determined by such influences as the need to isolate the instruments from vibration, or the need for instrument modularity. The external environment surrounding the transfer line as it extends from within one instrument to within another can vary significantly. For example, a transfer line traversing from the inside of a heated gas chromatograph oven, through the oven wall insulation and out to the ambient air gap between instruments experiences a variation in temperatures ranging from perhaps 20 to 350 degrees C.
Prior art transfer lines have been limited to the rather simple approach of inserting the gas capillary tube in comparatively large body of a highly thermal conductive metal housing which is heated by an enclosed heater, such as a cartridge or band heater. This combination is further surrounded by a thermal insulator of some type, which tends to mitigate changes in the surrounding environment. The larger the metal body and insulation, the more uniform the temperature. Several disadvantages are evident in this approach. First, since the mass of the metal housing is primarily designed to house the heater rather than the small glass tube, such heated transfer devices are particularly power inefficient and subject to substantial heat loss in the surrounding instrument. Analytical detectors such as spectrophotometers are particularly sensitive to temperature gradients due to the precise alignment of their optics and can not afford the heat dissipation from a relatively large transfer line operating at 350-400 degrees C., as is required in gas chromatography. Typically, these prior art transfer lines have diameters of 5 cm or more and due to their size, dissipate as much as several hundred watts of power into their host instruments. These large diameters also inhibit coupling of the transfer line to instrument devices. It is not uncommon for cold junctions to develop at the coupling which are 50 degrees C. or more below the desired operating point.
Alternatively, prior art transfer lines have consisted of a weave of highly electrically resistant wire, such as an alloy of 80% nickel and 20% chromium, and fibrous glass or ceramic insulation wound about the glass capillary tube into which the glass capillary may be inserted. Electrical current supplied to the metal wire heats the capillary. Although usually smaller in diameter than the previously described transfer lines, the woven transfer lines lack temperature uniformity when positioned in a temperature environment which changes along the length of the transfer line. This is because there is insufficient thermal conductivity in the axial direction of the capillary tube.
Other prior art transfer lines have consisted of a tubular structure heated at one end, the capillary tube passing through the center of the tube. The heating element is housed in metal attached to the tube and heat is transferred by conduction down the length of the tube. Although this approach provides for a smaller size on the non-heated end, it obviously suffers from non-uniform heating of the capillary tube. To maintain a minimum temperature at the cold end the heater must be brought to a higher temperature than required for the chemical analysis.